1.. SPDX-License-Identifier: GPL-2.0 2 3======================================= 4Linux ACPI Custom Control Method How To 5======================================= 6 7:Author: Zhang Rui <rui.zhang@intel.com> 8 9 10Linux supports customizing ACPI control methods at runtime. 11 12Users can use this to: 13 141. override an existing method which may not work correctly, 15 or just for debugging purposes. 162. insert a completely new method in order to create a missing 17 method such as _OFF, _ON, _STA, _INI, etc. 18 19For these cases, it is far simpler to dynamically install a single 20control method rather than override the entire DSDT, because kernel 21rebuild/reboot is not needed and test result can be got in minutes. 22 23.. note:: 24 25 - Only ACPI METHOD can be overridden, any other object types like 26 "Device", "OperationRegion", are not recognized. Methods 27 declared inside scope operators are also not supported. 28 29 - The same ACPI control method can be overridden for many times, 30 and it's always the latest one that used by Linux/kernel. 31 32 - To get the ACPI debug object output (Store (AAAA, Debug)), 33 please run:: 34 35 echo 1 > /sys/module/acpi/parameters/aml_debug_output 36 37 381. override an existing method 39============================== 40a) get the ACPI table via ACPI sysfs I/F. e.g. to get the DSDT, 41 just run "cat /sys/firmware/acpi/tables/DSDT > /tmp/dsdt.dat" 42b) disassemble the table by running "iasl -d dsdt.dat". 43c) rewrite the ASL code of the method and save it in a new file, 44d) package the new file (psr.asl) to an ACPI table format. 45 Here is an example of a customized \_SB._AC._PSR method:: 46 47 DefinitionBlock ("", "SSDT", 1, "", "", 0x20080715) 48 { 49 Method (\_SB_.AC._PSR, 0, NotSerialized) 50 { 51 Store ("In AC _PSR", Debug) 52 Return (ACON) 53 } 54 } 55 56 Note that the full pathname of the method in ACPI namespace 57 should be used. 58e) assemble the file to generate the AML code of the method. 59 e.g. "iasl -vw 6084 psr.asl" (psr.aml is generated as a result) 60 If parameter "-vw 6084" is not supported by your iASL compiler, 61 please try a newer version. 62f) mount debugfs by "mount -t debugfs none /sys/kernel/debug" 63g) override the old method via the debugfs by running 64 "cat /tmp/psr.aml > /sys/kernel/debug/acpi/custom_method" 65 662. insert a new method 67====================== 68This is easier than overriding an existing method. 69We just need to create the ASL code of the method we want to 70insert and then follow the step c) ~ g) in section 1. 71 723. undo your changes 73==================== 74The "undo" operation is not supported for a new inserted method 75right now, i.e. we can not remove a method currently. 76For an overridden method, in order to undo your changes, please 77save a copy of the method original ASL code in step c) section 1, 78and redo step c) ~ g) to override the method with the original one. 79 80 81.. note:: We can use a kernel with multiple custom ACPI method running, 82 But each individual write to debugfs can implement a SINGLE 83 method override. i.e. if we want to insert/override multiple 84 ACPI methods, we need to redo step c) ~ g) for multiple times. 85 86.. note:: Be aware that root can mis-use this driver to modify arbitrary 87 memory and gain additional rights, if root's privileges got 88 restricted (for example if root is not allowed to load additional 89 modules after boot). 90